1. Field of the Invention
This invention relates to magnetic resonance imaging techniques and, more particularly, to a method for selectively suppressing and/or exciting chemical components in tissue to modify a magnetic resonance image produced in response to a multiple shaped radiofrequency pulse sequence applied prior to the generation of a magnetic field gradient pulse sequence.
This invention further relates to a magnetic resonance imaging technique in which a multiple shaped radiofrequency pulse sequence manipulates the magnetization in the transverse plane of tissue being imaged in a manner which enhances the contrast among chemical components in the tissue.
2. Description of Related Art
Magnetism results from the motion of electric charges such as electrons. Electrons can produce a magnetic field either by motion along a path or by virtue of their intrinsic spin. The particles that comprise the atomic nucleus, collectively called nucleons, also have spin and magnetic moment. Because both individual nucleons and nuclei have a charge distribution, rotation or spin of this charge produces a magnetic dipole whose value is called a magnetic moment. The numeric value of the magnetic moment determines the energies of the different orientations of a nucleus in an external magnetic field. The proton is positively charged and has a relatively large magnetic moment. Although neutral, the neutron also has a net magnetic moment. A neutron's magnetic moment is about two-thirds of the value of the proton's and points against the axis of spin. When in the nucleus, like nucleons align with their spins pointing against each other. This phenomena is called "pairing" and is favored because it leads to a lower nuclear energy state. Therefore, only the unpaired, odd proton or neutron, or both, contribute their magnetic moment to the nucleus. As a consequence, only nuclei with odd numbers of protons or neutrons, or both, have a magnetic moment. The magnetic properties of nuclei become important when they are placed in external magnetic fields as the nuclei will have a tendency to align with the external field.
Resonance occurs when an amount of energy equal to the difference of energy associated with the transition between states is absorbed or released. In the case of a magnetic moment of a nucleus, transitions between parallel or "up" and anti-parallel or "down" states can occur if the correct amount of energy is absorbed or released. Because the interaction is with a magnetic element, the necessary energy can be provided by a magnetic field. One way to obtain such a field is by utilizing electromagnetic radiations. To induce resonance, the frequency f of the electromagnetic radiation must be proportional to the local magnetic field H.sub.L. The particular proportionality constant which will induce resonance varies depending on the particular nucleus involved. The relationship between frequency and field is given by: EQU f=(gamma) H.sub.L /2(pi) (1)
where (gamma) is the magnetogyric ratio of the nucleus.
When the nuclei, originally in equilibrium with the field, are irradiated at the resonant frequency, the nuclei can adopt the anti-parallel state. When the return to equilibrium, if the field is unchanged, they will radiate emissions of the same frequency. If between excitation and radiation the field strength is changed, the nuclei will radiate a frequency corresponding to the new field value. This behavior of nuclei may be described by net magnetization vector N which characterizes the system by disregarding the state of each nucleus and considers only the net collective effect. In a magnetic field, the magnetization vector points along the field. The length of the magnetization vector is proportional to the number of nuclei in the sample and to the field strength and is inversely proportional to temperature. The length and direction of this vector characterizes the equilibrium magnetization of the sample; that is, the state that it will revert to after being disturbed if enough time is allowed to pass. This equilibrium magnetization is given by: EQU (mu).sup.2 H/kT (2)
where:
(mu) is the nuclear magnetic moment; PA1 k is Boltzmann's constant; and PA1 T is the absolute temperature.
This vector can be disturbed from equilibrium by the application of a second external magnetic field. If such a field is superimposed upon the first magnetic field, M will align with the new net field. As M moves to its new direction, energy stored in the nuclei of the sample is provided by the second field. When the superimposed field is removed, M returns to equilibrium and the nuclei release the stored energy to the environment, either as heat or RF energy. These two fields are called the transverse field and the longitudinal field, respectively. More specifically, the component of M that points along the main field is called the longitudinal magnetization (M.sub.L) and the orthogonal component is called the transverse magnetization (M.sub.T). If the transverse field is an RF field at the resonant frequency, M behaves as a top such that, as it deviates from the longitudinal axis, it precesses about it. If the main magnetic field is defined as being aligned along the z axis, then M.sub.T rotates in the x, y plane and M.sub.L is reduced from its equilibrium value. If M is rotated onto the x, y plane by a 90 degree RF pulse, M.sub.L is 0.
Immediately after an RF irradiation, M.sub.L begins to grow again towards its equilibrium value M. This growth is exponential with a time constant T1 such that: EQU M.sub.L =M[1-exp(-t/T1)] (3)
where t is the time since irradiation.
During this process, M.sub.T decays exponentially with a time constant T2 such that: EQU M.sub.T =M.sub.TO exp(-t/T2) (4)
where: M.sub.TO is the value of M.sub.T immediately after irradiation; and t is the lapse time.
When a proton is aligned with the magnetic field, it gives off no signal. When a proton is perpendicular to the field, it gives off a maximum signal. The rate at which a proton realigns with the static field is called its "T1" or "T1 relaxation time". The T1 relaxation time is also called "spin-lattice" or "thermal relaxation time". The individual protons exchange fixed amounts of energy when they flip from the down to up alignment in the process of returning to equilibrium. This exchange can occur only at the resonant frequency. A molecule in the lattice surrounding the resonant nucleus appears as an oscillating electric magnetic field with frequency that depends on its thermal velocity and mean free path. Since both vary over a broad range for any one temperature, of the whole ensemble of molecules, only a small fraction provide the right oscillating fields. These then coupled with the nucleus and allow the relaxation to occur. As temperature and molecular composition changes so does the distribution of velocities and mean free paths, thus affecting T1.
When a group of protons precess in phase, the voxel gives off a maximum signal. When a group of protons precess out of phase, the voxel gives off no signal. The rate at which the protons de-phase is called its "T2" or "T2 relaxation time". The T2 relaxation time is also called the "spin-spin" or "transverse relaxation time". In a perfectly uniform magnetic field, all nuclei will resonate at exactly the same frequency, but if the field is even slightly inhomogeneous, nuclei resonate at slightly different frequencies. Although immediately after an RF irradiation, the nuclei are all in phase, they soon lose coherence and the signal that is observed, decays. Any such loss of coherence shortens T2. Thus, the effects due to inhomogeneities in the external field produce a rapid decay characterized by the relaxation time T2.
Magnetic resonance has become an established method for producing an image of the internal structure of an object. Such methods have numerous applications particularly in medical diagnostic techniques. For example, the examination and diagnosis of possible internal derangements of the knee is one such application of magnetic resonance imaging techniques. Most magnetic resonance techniques for knee imaging use a two-dimensional (or "2D") acquisition with a spin-echo pulse sequence to provide T1, T2 and proton density weighted images of the knee in multiple planes, typically the sagittal (y-z) and coronal (x-z) planes. However, the selective excitation techniques used by conventional 2D methods is limited in the ability to obtain thin slices by the gradient strength of the system. Furthermore, obtaining images in non-orthogonal planes is often advantageous for proper medical diagnosis. However, to obtain images in a non-orthogonal plane, the use of 2 gradients rather than a single gradient is required to obtain a slice. Finally, oblique plane imaging of an object requires a corrected procedure after obtaining each gradient echo to keep the slices passing through the object being imaged.
As a result of the shortcomings of 2D methods, three dimensional (or "3D") acquisitions of magnetic resonance data has been used to produce thin slice, high resolution images. See, for example, the publications to Steven E. Harms, "Three-dimensional and Dynamic MR Imaging of the Musculoskeletal System"; Harms and Muschler, "Three-Dimensional MR Imaging of the Knee Using Surface Coils", Journal of Computer Assisted Tomography; 10(5): 773-777 (1986) and Sherry et al., (Spinal MR Imaging: Multiplanar Representation from a Single High Resolution 3D Acquisition", Journal of Computer Assisted Tomography; 11(5): 859-862 (1987); Robert L. Tyrrell, "Fast Three-dimensional MR Imaging of the Knee: Comparison with Arthroscopy", Radiology; 166: 865-872 (1988); Charles E. Spritzer, et al., "MR Imaging of the Knee: Preliminary Results with a 3DFT GRASS Pulse Sequence", American Journal of Roentology; 150:597-603 (1987); Alan M. Haggar, et al., "Meniscal Abnormalities of the Knee: 3DFT Fast-Scan GRASS MR Imaging", American Journal of Roentology; 150:1341-1344 (1988).
Also disclosed in the article to Steven E. Harms entitled "Three-dimensional and Dynamic MR Imaging of the Musculoskeletal System" is a pulse sequence referred to as a "fast adiabatic trajectory in steady state" or "FATS" pulse sequence. The FATS RF pulse sequence is characterized by the utilization of non-selective opposing 20 degree adiabatic half-passage pulses that result in no effective magnetization on resonance. Because the sequence is tuned to fat, water signal is detected off resonance. See also the article entitled "Method combines vascular, anatomic data", Diagnostic Imaging, pgs. 140-42 (February, 1991). While the FATS type RF pulse sequence tends to suppress the fat signal, the FATS type RF pulse sequence tends to have a relatively narrow null bandwidth, thereby decreasing the effectiveness of the fat suppression since off resonance fat would not be suppressed.